We live in a post-modern world of "live and let live". There is no sense of absolute values. But we believe that there ARE absolute values. So if someone is, for example, a mechalel Shabbos, and he knows better - this is what he is doing:
1] He is bringing suffering not only to his soul but to the world at large. Sin brings suffering. That is a principal of faith.
2] He is desecrating the memory of all of those who sacrificed everything to keep Shabbos - even when it meant being sent to Siberia for 20 years of forced labor. Or in America, the loss of a livelihood. When one flippantly and cavalierly violates Shabbos he is essentially saying that all of their suffering was silly and for no good reason.
3] He is engaging in an act of gross ingratitude. Hashem gave him life and trillions of gifts every second [including every healthy cell] and asks in return that he keep Shabbos. Can one be an moral person and not keep Shabbos? Of course not. There is nothing less moral than being a משיב רעה תחת טובה - giving back evil in exchange for all of the good someone did on your behalf. There is no greater "Someone" than Hashem.
4] He has the halachic status of a goy in certain areas [one can't drink wine he moves etc.].
5] He causes tremendous emotional anguish to those who know that he is not keeping Shabbos - especially religious family members such as parents. He causes the Shechina pain.
6] He is not keeping one of the ten commandments which are the basis of the whole Torah.
7] Chazal say that in the merit of keeping Shabbos, Moshiach will come. it is logical to say that not keeping Shabbos pushes off his arrival. So such a person has a hand in delaying the redemption of the world.
NOTE: This is not a kiruv post. One should not tell irreligious people how horrible they are. That is mekarev nobody. We have to be mekarev them with love ["עבותות אהבה" as the Navi puts it] but not by making them our heroes. I wrote in continuing the theme we have been expanding upon that non-observant Jews who are willful, fully conscious sinners, particularly those raised frum, don't deserve special respect or a forum for spreading their ideas [such as inviting them to speak]. We owe it to them and to those in the sphere of our influence to make it clear that we have obligations in life and that it is an affront to G-d and the entire Jewish people to willfully flout those obligations.
